Cheers, all!
cudak888 I was actually still out on the first ride when I posted from my phone - I came home and added some pertinent details to the initial post!
Charles Wahl The Simplex Tour de France derailleur shifts smoothly and I happened to luck out and get the shifter tightened just right on the first go! It'll sit in the top cog if I put it there by hand but it's got very minimal space between where it should be and the spokes, so I'm waiting for initial cable stretch before I re-tune it. Currently I'm running with the three smaller cogs and it moves very daintily between them.
Getting the shifter set up with two hands was a bear, so sharing how I did that seems like a reasonable thing. At rest it was just touching the spokes, so I had to adjust it in slightly. In the end I compressed the spring with one hand, pulled out the chain and then wrapped the thing with a layer of duct tape to hold it in position while I threaded and tightened up the cable. Also, this one was missing the cable nut with the eyelet, so I took one off a side-pull brake caliper and used some washers to make it fit correctly, as can be seen in the photo.
-Gregory